X-linked lymphoproliferative disease (XLP) is an inherited immunodeficiency syndrome characterized by increased susceptibility to Epstein-Barr virus (EBV) infection. Rather than transient infectious mononucleosis after primary exposure to EBV, 40-50% of boys with XLP experience fatal uncontrolled lymphoproliferation and 20-30% develop EBV-associated lymphoma. As such, XLP represents a genetic model to study mechanisms involved in EBV-associated lymphoproliferation and tumorigenesis. XLP also has clinical relevance to EBV-associated lymphomas seen in patients with HIV infection or those immunosuppressed after bone marrow or solid organ transplantation. The goal of this project is to identify and characterize the gene for XLP. Through RFLP analysis, XLP has been linked to the locus Xq25 and to date, 3 unrelated patients have been reported sharing cytogenetic deletions at this locus. To determine the affected gene, a positional cloning strategy will be used involving the following approaches: (1) To screen arrayed YAC libraries with known and novel genetic markers at the XLP locus and to generate a linear contig of YACs spanning the XLP deletion, (2) To identify coding sequences contained within these YACs by generating a cosmid library from YAC DNA and then either identifying exons through the "exon amplification" technique, or cDNA clones through the process of "direct selection", (3) To characterize transcripts through evaluation of homology to other known genes, pattern of expression among tissues and conservation between species, and to confirm that identified transcripts are the involved gene by performing mutational analysis of patient samples, (4) To evaluate properties of the XLP gene product through examination of subcellular localization, effects on cell growth and proliferation, association with other proteins, and generation of possible animal models. Cloning the XLP gene will provide a better understanding of the mechanisms through which EBV induces lymphoma and through which the host recognizes and responds to EBV-infected cells. Characterizing properties of the XLP gene product may provide clues to therapeutic strategies for these and other human diseases associated with EBV.